In a typical electricity meter measurement environment, the amount of electric current or power consumed can vary by as much as a hundred times as different appliances are switched on an off. One type of digital electricity meter has a high resolution ADC with enough resolution to measure the current signal under the full range of variation and yet keep within the required accuracy specifications. However, since high resolution ADCs are expensive, “auto-ranging” techniques were developed that use an external amplifier together with a lower resolution ADC to achieve the same accuracy requirement at lower cost.
In auto-ranging, as the amount of electric current decreases, the current signal will get weaker. In response, the auto-ranging circuit will increase the gain of an amplifier to boost the current signal before it goes into the A/D converter. This keeps the amplifier's output range dynamically matched with the A/D converter's input range so that even under weak current conditions the A/D output will be in the proper range. The system tracks the gain used by the amplifier and scales the results accordingly. For example, a 60 A current gives an A/D output of 0-200 with an 8-bit ADC. When the current drops to 15 A (one quarter of its original value), in an auto-ranging system, the gain of the amplifier is increased by 4 times so that the signal looks like a 60 A signal to the A/D and it still output between 0-200 at 8 bit resolution. So effectively, the dynamic range is (0-200)×4 which is the same as that of a 10 bit ADC.
The ideal situation is to perform auto-ranging on every sample, however, system limitations, like the gain switching speed of the amplifier and the stability of the circuit etc., limits the auto-ranging to blocks of samples. Typically, this block consists of thousands of samples which take about 0.5 seconds to 1 second to complete.
FIG. 1 illustrates a problem with prior art auto-ranging systems. Because the prior art circuit processed the data in blocks, if the amount of current consumed suddenly increases from a very small amount to a very large amount 10 during a block, then it is quite possible that the gain setting of the amplifier is so large that the amplifier goes into saturation 12 (limited output). When the amplifier goes into saturation, the A/D reading will be inaccurate for the saturation portion of the block. The reading will be incorrect until the system re-adjusts the gain for a subsequent block 14 to prevent the saturation 16. In certain types of appliances like an electric water heater, this sudden increased current consumption can happen in bursts as the thermostat switches on and off continuously and therefore the auto-ranging system can yield very inaccurate results.
FIG. 1a shows the 1st block suffers from error before the gain is adjusted. FIG. 1b shows the second block has the correct gain setting.